Natural gas vehicles (NGVs) operate on the same basic principles as other internal combustion-powered vehicles. Fuel, in the form of natural gas, is mixed with air and fed into a cylinder where the mixture is ignited to move a piston up and down. Natural gas can power vehicles currently powered by gasoline and diesel fuels. However, at standard temperature and pressure, natural gas is a gas rather than a liquid. This gives rise to two types of NGVs: those that are configured to use compressed natural gas (CNG) and those that are configured to operate on liquid natural gas (LNG).
CNG is typically stored on-board a vehicle under high pressure (3,000-3,600 pounds per square inch) in cylindrical containers that attach to the top, rear, or undercarriage of the vehicle.
Fueling CNG vehicles occurs at CNG stations, where natural gas is typically supplied from a local gas utility line at low pressure. There are two types of fueling systems typical employed for CNG refueling: fast-fill systems and time-fill (or slow-fill) systems. Fast-fill systems typically require a large volume high-pressure storage tank system. Such systems therefore have a significantly large footprint, and require a relatively complex and costly instillation. Slow-fill systems take a longer time to refuel a vehicle compared to fast-fill systems, but do so by providing CNG to the vehicle from a compressor fed by a low-pressure gas utility line. Because slow-fill systems typically do not utilize a large volume high-pressure storage tank, they can have a much smaller footprint, thus increasing flexibility in locations available for refueling stations.